Recent advances in bipolar/BiCMOS IC design and fabrication make possible the integration of digital and analog circuits on the same IC chip. This technology is widely used in mobile communication systems where a digital core is combined with analog RF circuits. The digital and analog circuits are typically formed as a variety of components.
To improve the performance of the CMOS devices, lightly doped drain/source (LDD) regions are formed. For digital CMOS devices, which are typically core devices, pocket regions are also formed in order to increase the junction abruptness. FIG. 1 illustrates a conventional NMOS digital device comprising LDD regions 2 and pocket regions 4. LDD regions 2 are of n-type, while pocket regions 4 are of p-type. Typically, LDD regions 2 are implanted using gate electrode 6 as a mask and are substantially aligned with edges of the gate electrode 6. Pocket regions 4 are typically slant implanted in order to extend under the gate electrode 6. Source/drain regions 8 are formed using spacers 10 as masks. PMOS devices have similar structures, and are formed using similar methods with respective impurity types reversed.
In the above-discussed processes, two masks, a first mask and a second mask, are required. The first mask is for masking NMOS devices when LDD and pocket regions of PMOS devices are formed. The second mask is for masking PMOS devices when respective LDD and pocket regions of NMOS devices are formed. Since each mask is used for the formation of both LDD and pocket regions, the formation of LDD regions and pocket regions are coupled.
The masks for the formation of digital CMOS devices, however, are not suitable for the formation of analog devices, which are typically used in I/O circuits. As opposed to digital CMOS devices that have only an “on” and an “off” state, analog CMOS devices have to operate between the two states and are more sensitive to impurity distribution. It has been found that analog CMOS devices are particularly sensitive to pocket doping. The formation of the pocket regions significantly lowers the intrinsic gain of the analog CMOS devices. Additionally, the threshold voltage of the analog CMOS devices becomes hard to control, thus making device matching more difficult when pocket regions are formed. This creates problems for the design of certain analog circuits, such as differential amplifiers. Therefore, it is preferred that analog devices not have pocket regions.
As the LDD regions and pocket regions of CMOS devices are coupled, if the same masks are used for the formation of pocket regions of digital devices, pocket regions will also be formed simultaneously for analog devices. As a solution, the first and the second masks are modified to be used for the formation of digital MOS devices only, with analog device regions masked. Two more masks, a third and a fourth mask, are added, wherein the third mask is for the formation of LDD regions of analog PMOS devices, and the fourth mask is for the formation of LDD regions of analog NMOS devices. The third and the fourth masks mask the digital CMOS regions.
Using the first, the second, the third and the fourth masks, each of the digital devices may have both LDD regions and pocket regions, while each of the analog devices only have LDD regions. However, the conventional four-mask scheme is not flexible, making further improvement of the device performance difficult. Additionally, the intrinsic gains of the analog MOS devices, which are important factors for the performance of analog devices, cannot be tuned. Therefore, more flexible LDD and pocket formation schemes are needed.